"What's coming—another storm?" asked Ralph.
"I don't know what it is yet," rejoined the other in a strangely uneasy tone, "it looks like—like——"
"A pillar of dust," exclaimed Jack, who had by this time sighted it, too, and had come to the aid of the unimaginative plainsman.
"So it does," cried the others, who now, with the exception of the short-sighted professor, could also see the approaching dust-cloud.
"What can it be?" wondered Walt, peering eagerly in its direction.
"Somebody riding. Several of 'em, I should say, by the dust they're raising," rejoined Pete bluntly.
The boys exchanged quick glances. Somebody riding across that arid waste? Their destination could only be the mesa, then, but who could it possibly be?
Had they been able to solve the riddle at that instant, they would have scattered pell-mell for their ponies, and made the best of their way from the Haunted Mesa, but, not being endowed with anything more than ordinary sensibilities, it was, of course, impossible for them to realize the deadly peril that was bearing down upon them in that dust-cloud.
"I can see things more clearly now," cried Jack, as for an instant a vagrant desert air blew aside the dust-cloud and revealed several riders, surrounding some cumbersome, moving object in their midst.
"There's a wagon!" he cried, "a big one, too, and surrounded by horsemen. What can it mean?"